The present invention is directed to a method for producing an optical fiber cable whose sheath is drawn from an extruder with an adjustable haul-off rate to loosely contain a core of at least one of the light waveguide so that by guidance of the optical fiber cable around at least one deflection roller following the extruder, the core forms a definite one-sided seating surface together with the inside wall of the sheath in order to obtain a defined shrinkage of the sheath during subsequent cooling.
A method of this type is disclosed by European published patent application No. 01 93 940, which claims priority from the same German application as allowed U.S. patent application Ser. No. 834,768, filed Feb. 28, 1986, which issued on Mar. 1, 1988, as U.S. Pat. No. 4,728,470. and whose disclosure is incorporated by reference. In this allowed application, a certain light waveguide length is set by adequate coupling of the light waveguide to the sheath and the coupling between the light waveguide sheath is thereby achieved throughout by looping the cable over a roller or disc with a large radius and the loop is about 180.degree. and optimally at least 360.degree.. The light waveguide is then uncoupled from the influences of the transmission side and its length is defined by revolutions around the wheel or by running onto dishes. When the sheath at this location has temperature deviating from a standardized, ambient temperature, a length modification of the sheath will occur and, as a result thereof, a definite overlength or underlength (given over-cooling of the sheath) is achieved by subsequent temperature equalization. The negative deviation of the light waveguide from the sheath length on a large radius is either left out of the consideration or is considered to be a side-effect.
The method initially described, however, suffers from a series of disadvantageous incidental influences which can considerably influence the final results. Thus, for example, the fiber braking tension, which is supposed to hold the light waveguide taut up to the take-up dish can be reduced by unavoidable, preceding deflections and can even be rendered ineffective under certain conditions. These deflections then determine the decoupling of the fiber braking tension and, thus, the final light waveguide lengths due to the temperature effective there. In addition, negative length deviations of the light waveguide varies when the length-defining quantity is the variable seating diameter of the dish. In addition, a sheath haul-off force is required due to the injection or extrusion method which is used for manufacturing of the loose sheath, and this sheath haul-off force is maintained until haul-off and stretching of the sheath in an elastic-plastic fashion has occurred. The force disappears following the haul-off, the sheath subsequently will shorten. Excess lengths which are dependent not only on the temperature differences, but also on the haul-off rate and on the extrusion temperatures can thus occur. In addition, the filling compound introduced at the extruder by means of a special method has a highly temperature-dependent minimum shearing strain and exceeding this is what makes a sliding of the light waveguide in the compound possible. This shearing strain must be exceeded by the light waveguide braking force in every case. When, even given a stretched light waveguide, the length-defining radius, for example of the dish, is too far from the extruder, the braking force may potentially no longer suffice for pulling the light waveguide taut up to the length-defining element. This situation can hardly be completely controlled.
It is expedient that the length definition or determination be, in fact, undertaken at a suitable equilibrium temperature, and as close as possible to the extruder. In addition, the length definition should not occur on a dish having a varying coiling diameter. When deflection occurs, they must either lead to a reliable coupling of the light waveguide to the optical fiber cable or must be suppressed. Deflections having an unclear cling factor are to be avoided, at least preceding the location of the length definition of the light waveguides. Substantial temperature variations ae also to be avoided, at least in the region of the length-defining element. Finally, an unavoidable elastic-plastic sheath shortening of the light waveguide must be compensated or over-compensated so that every desired over-length or under-length of the light waveguide and the optical fiber cable sheath can be basically set for every speed and every light waveguide dimension. This compensation should be adjusted and, as experience has shown, should be adapted to the speed utilized in the process.